FAQ on UMTS Idle Mode Procedures

By Zahid Ghadialy (zahidtg@yahoo.com)

Last Updated: 25/06/2006

How does PLMN Selection take place in UMTS?

The UE normally operates on its home PLMN (HPLMN) or equivalent home PLMN (EHPLMN). However a visited PLMN (VPLMN) may be selected, e.g., if the MS loses coverage. There are two modes for PLMN selection:

Automatic mode: This mode utilizes a list of PLMNs in priority order. The highest priority PLMN which is available and allowable is selected.

Manual mode: Here the MS indicates to the user which PLMNs are available. Only when the user makes a manual selection does the MS try to obtain normal service on the VPLMN.

There are two cases:

International Roaming: This is where the MS receives service on a PLMN of a different country than that of the HPLMN.

National Roaming: This is where the MS receives service from a PLMN of the same country as that of the HPLMN, either anywhere or on a regional basis. The MS makes a periodic search for the HPLMN while national roaming.

To prevent repeated attempts to have roaming service on a not allowed LA, when the MS is informed that an LA is forbidden, the LA is added to a list of "forbidden LAs for roaming" which is stored in the MS. This list is deleted when the MS is switched off or when the SIM is removed. Such area restrictions are always valid for complete location areas independent of possible subdivision into GPRS routing areas. The structure of the routing area identifier (3GPP TS 23.003) supports area restriction on LA basis.

If a "No Suitable Cells In Location Area" message is received by an MS, that location area is added to the list of "forbidden LAs for roaming" which is stored in the MS. The MS shall then search for a suitable cell in the same PLMN but belonging to an LA which is not in the "forbidden LAs for roaming" list.

If a "PLMN not allowed" message is received by an MS in response to an LR request from a VPLMN, that VPLMN is added to a list of "forbidden PLMNs" in the SIM and thereafter that VPLMN will not be accessed by the MS when in automatic mode. A PLMN is removed from the "forbidden PLMNs" list if, after a subsequent manual selection of that PLMN, there is a successful LR. This list is retained when the MS is switched off or the SIM is removed. The HPLMN (if the EHPLMN list is not present or is empty) or an EHPLMN (if the EHPLMN list is present) shall not be stored on the list of "forbidden PLMNs".

In A/Gb mode, an ME not supporting SoLSA may consider a cell with the escape PLMN code (see 3GPP TS 23.073) to be a part of a PLMN belonging to the list of "forbidden PLMNs".

Optionally the ME may store in its memory an extension of the "forbidden PLMNs" list. The contents of the extension of the list shall be deleted when the MS is switched off or the SIM is removed.

If a "GPRS services not allowed in this PLMN" message is received by an MS in response to an GPRS attach, GPRS detach or routing area update request from a VPLMN, that VPLMN is added to a list of "forbidden PLMNs for GPRS service" which is stored in the MS and thereafter that VPLMN will not be accessed by the MS for GPRS service when in automatic mode. This list is deleted when the MS is switched off or when the SIM is removed. A PLMN is removed from the list of "forbidden PLMNs for GPRS service" if, after a subsequent manual selection of that PLMN, there is a successful GPRS attach. The maximum number of possible entries in this list is implementation dependant, but must be at least one entry. The HPLMN (if the EHPLMN list is not present or is empty) or an EHPLMN (if the EHPLMN list is present) shall not be stored on the list of "forbidden PLMNs for GPRS service".

In the UE, the AS shall report available PLMNs to the NAS on request from the NAS or autonomously.

UE shall maintain a list of allowed PLMN types. The allowed PLMN type can be GSM-MAP only, ANSI-41 only or both. During PLMN selection, based on the list of allowed PLMN types and a list of PLMN identities in priority order, the particular PLMN may be selected either automatically or manually. Each PLMN in the list of PLMN identities can be identified by either 'PLMN identity' (GSM-MAP) or 'SID'. In the system information on the broadcast channel, the UE can receive a 'PLMN identity' (GSM-MAP) or a 'SID' or a 'PLMN identity' (GSM-MAP) and a 'SID', in a given cell. For a given cell, the UE might receive several 'PLMN identities' from the system information on the broadcast channel. The result of the PLMN selection is an identifier of the selected PLMN, the choice being based on the allowed PLMN types, UE capability or other factors. This identifier is one of either 'PLMN identity' for GSM-MAP type of PLMNs or 'SID' for ANSI-41 type of PLMNs.

On request of the NAS the AS should perform a search for available PLMNs and report them to NAS.

The UE shall scan all RF channels in the UTRA bands according to its capabilities to find available PLMNs. On each carrier, the UE shall search for the strongest cell and read its system information, in order to find out which PLMN the cell belongs to. If the UE can read one or several PLMN identities in the strongest cell, each found PLMN shall be reported to the NAS as a high quality PLMN (but without the RSCP value), provided that the following high quality criterion is fulfilled:

For an FDD cell, the measured primary CPICH RSCP value shall be greater than or equal to -95 dBm.

For a TDD cell, the measured P-CCPCH RSCP shall be greater than or equal to -84 dBm.

Found PLMNs that do not satisfy the high quality criterion, but for which the UE has been able to read the PLMN identities are reported to the NAS together with the CPICH RSCP value for UTRA FDD cells and P-CCPCH RSCP for UTRA TDD cells. The quality measure reported by the UE to NAS shall be the same for each PLMN found in one cell.

The search for PLMNs on the rest of the carriers may be stopped on request of the NAS. The UE may optimise this search by using stored information of carrier frequencies and optionally also information on cell parameters, e.g. scrambling codes, from previously received measurement control information elements.

Once the UE has selected a PLMN, the cell selection procedure shall be performed in order to select a suitable cell of that PLMN to camp on.

The initial cell-selection procedure is used in case there is no information on the current environment stored in the UE. However, normally the UE starts the cell selection with a stored information cell-selection procedure. The UE may have stored the necessary information of the cell it was previously camped on, such as frequency and scrambling code. The UE may first try to synchronize into that cell, and if it fails, it may trigger the initial cell selection.

The purpose of the initial cell-selection procedure is to find a cell, not necessarily the best cell, but a usable cell, for the UE to camp on after power-on. In the UTRAN, the number of carrier frequencies is quite small. One operator typically operates only on two or three frequency carriers. In the first phase of UMTS in Europe, the frequency allocation for UMTS-FDD is 2 × 60 MHz (uplink/downlink), which means that there can be, at most, only 12 carrier frequencies of 5-MHz bandwidth each. These carriers are then divided between up to six operators. Each carrier will only support one operator. This obviously forces the operators to coordinate their networkplanning activities near national borders because the same frequency can be used by different operators in adjacent countries.

The specifications do not accurately dictate how the initial cellselection procedure should be implemented; it is left for the UE manufacturers to decide. Most of the functionality, however, has to be in the physical layer, and the RRC layer has only a management role. The initial cell-selection procedure is performed on one carrier frequency at a time until a suitable cell is found. In principle the process includes the following:

Search for primary synchronization channels (P-SCHs);

Once such a channel is found, acquire time-slot synchronization from it;

Acquire frame synchronization from the corresponding S-SCH;

Acquire the primary scrambling code from the corresponding CPICH;

Decode system information from the cell to check whether it is a suitable cell for camping (i.e., it contains the right PLMN code and access to it is allowed).

All P-SCHs have the same fixed primary synchronization code. The search procedure should yield a set of P-SCHs in the area. Because the P-SCH is only transmitted during the first 256 chips of each time slot, the beginning of its transmission also indicates the start of a time slot in the corresponding cell.

In the second phase of the process, the received signal is correlated with all possible secondary synchronization code (S-SCH) words on the S-SCH. There are 16 different SSCs, and these can be combined into 64 different code words, each with a length of 15 SSCs. Once the right code word is found, this gives the UE the frame synchronization and the code group identity, which indicates eight possible primary scrambling codes for the control channels.

The third phase of the procedure consists of finding the right primary scrambling code for this cell. Each candidate cell’s primary scrambling code (there are eight of them as shown in the second phase) is applied, in turn, to the common pilot channel (CPICH) of that cell. Because the CPICH carries a predefined bit/symbol sequence, the UE knows when it has found the correct primary scrambling code. The resolved primary scrambling code can then be used to detect the CCPCH, which carries the BCH, which contains the system information the UE is seeking. There are various ways to optimize this procedure to make it quicker. Note that phase five actually contains another major procedure, PLMN (i.e., the operator) selection. PLMN is identified by a PLMN code, a number that is transmitted on the BCCH channel of that network. A UE tries to find its home PLMN, the operator it has a contract with. In principle, a UE should first scan through all UTRAN frequencies until a good PLMN is found, and then start an initial cell-selection process on that frequency.

Note that one frequency can only be used by one operator (except in areas near country borders). However, while looking for the right PLMN code, the UE has already obtained all the necessary information for camping on a suitable cell, and no new scanning procedure is necessary once the correct PLMN is found. The situation is different if the UE is roaming abroad, and the home PLMN is not found. In that case RRC has to report all available PLMNs to NAS and wait for its selection decision, which can be either automatic or manual (user selection). This is time consuming, and many readers may have noticed this phenomenon when arriving at an airport in a new country and switching their GSM phones on. It may take a very long time before the phone registers to a network, especially if the phone is a multimode model with several frequency bands to scan.

The initial cell-selection process is repeated as many times as necessary until the first suitable cell is found for camping. Once the UE has managed to camp on a cell, it decodes the system information from it, including the neighbor cell list. This information can be used to help the UE find the best cell to camp onto. Note that the initial cell-selection procedure only found a cell to camp on (the first possible cell). It is possible that this cell will not be the best possible cell. For example, there could have been other frequencies including better cells for this particular UE that had not yet been scanned.

The neighbor cell list immediately tells the UE which frequencies and neighbor cells should be checked while the best possible cell is being searched for. The list includes additional information that can be used to optimize the cell-synchronization procedure, information such as the primary scrambling codes and timing information (optional, relative to the serving cell). With this information it should be possible to quickly descramble the CPICH from a neighbor cell.

From the CPICH it is possible to calculate the received chip energy to- noise ratio (Rx Ec/No) for this cell. This measurement is acquired for each neighbor cell in the list. Based on this information, the UE can determine whether there are better cells available. From a possible candidate cell, the UE must decode the system information to check that it is not barred for access.

If the neighbor cell list contains cells from another RAT—for example, GSM cells—and the serving cell quality level is worse than the Ssearch parameter, then the GSM cells must be taken into consideration in the cell reselection procedure.